Radical-based Advanced Oxidation Processes Research Articles

Comprehensive Insight into the Common Organic Radicals in Advanced Oxidation Processes for Water Decontamination.

Radical-based advanced oxidation processes (AOPs) are among the most effective technologies employed to destroy organic pollutants. Compared to common inorganic radicals, such as •OH, O2•-, and SO4•-, organic radicals are widespread, and more selective, but are easily overlooked. Furthermore, a systematic understanding of the generation and contributions of organic radicals remains lacking. In this review, we systematically summarize the properties, possible generation pathways, detection methods, and contributions of organic radicals in AOPs. Notably, exploring organic radicals in AOPs is challenging due to (1) limited detection methods for generated organic radicals; (2) controversial organic radical-mediated reaction mechanisms; and (3) rapid transformation of organic radicals as reaction intermediates. In addition to their characteristics and reactivity, we examine potential scenarios of organic radical generation in AOPs, including during the peroxide activation process, in water matrices or with coexisting organic pollutants, and due to the addition of quenching agents. Subsequently, we summarize various methods for organic radical detection as reported previously, such as electron paramagnetic resonance spectroscopy (EPR), 31P nuclear magnetic resonance spectroscopy (31P NMR), liquid/gas chromatography-mass spectroscopy (GC/LC-MS), and fluorescence probes. Finally, we review the contributions of organic radicals to decontamination processes and provide recommendations for future research.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes.

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Insights into the carbon nanotubes-mediated activation of permanganate for decontamination under high salinity

Radical-based advanced oxidation process (AOPs) has attracted great interests in wastewater treatment field. However, by the traditional radical-based method, the degradation of organic pollution is greatly suppressed when radicals react with the co-existing anions in the solution. Herein, an efficient method for degrading of contaminant under high salinity conditions is discussed through a non-radical pathway. Carbon nanotubes (CNTs) was employed as an electron transfer medium to facilitate the electron conversion from contaminants to potassium permanganate (PM). Based the results of quenching experiments, probe experiments, and galvanic oxidation process experiments, the degradation mechanism of CNTs/PM process was demonstrated to be electron transfer, rather than reactive intermediate Mn species. As a result, typical influencing factors including salt concentration, cations, and humic acid have less of an impact on degradation during CNTs/PM processes. In addition, the CNTs/PM system exhibits superior reusability and universality of pollutants, which has the potential to be applied as a non-radical pathway for the purification of contaminant in the large-scale high salinity wastewater treatment.

Distinguishing homogeneous advanced oxidation processes in bulk water from heterogeneous surface reactions in organic oxidation

Clarifying the reaction pathways at the solid-water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn3+ and MnOX) and Fenton and Fenton-like catalytic oxidation (e.g., Fe2+ and FeOCl catalyzing H2O2, Co2+ and Co3O4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid-water interface, which could guide the design of heterogeneous nanocatalysts.

Persulfate Process Activated by Homogeneous and Heterogeneous Catalysts for Synthetic Olive Mill Wastewater Treatment

Wastewaters from the olive oil industry are a regional environmental problem. Their phenolic content provides inherent toxicity, which reduces the treatment potential of conventional biological systems. In this study, Sulfate Radical based Advanced Oxidation Processes (SRbAOPs) are compared with advanced oxidation processes (namely Fenton’s peroxidation) as a depuration alternative. Synthetic olive mill wastewaters were submitted to homogeneous and heterogeneous SRbAOPs using iron sulfate and solid catalysts (red mud and Fe-Ce-O) as the source of iron (II). The homogenous process was optimized by testing different pH values, as well as iron and persulfate loads. At the best conditions (pH 5, 300 mg/L of iron and 600 mg/L of persulfate), it was possible to achieve 39%, 63% and 37% COD, phenolic compounds and TOC removal, respectively. The catalytic potential of a waste (red mud) and a laboratory material (Fe-Ce-O) was tested using heterogenous SRbAOPs. The best performance was achieved by Fe-Ce-O, with an optimal load of 1600 mg/L. At these conditions, 27%, 55% and 5% COD, phenolic compounds and TOC removal were obtained, respectively. Toxicity tests on A. fischeri and L. sativum showed no improvements in toxicity from the treated solutions when compared with the original one. Thus, SRbAOPs use a suitable technology for synthetic OMW.

Activation of peroxymonosulfate by novel Pt/Al2O3 membranes via a nonradical mechanism for efficient degradation of electron-rich aromatic pollutants

Novel flexible Pt/Al2O3 membranes were prepared and applied as catalysts for the degradation of organic pollutants in the presence of peroxymonosulfate (PMS). 100% of bisphenol A (BPA) could be removed within 20 min by using fresh Pt/Al2O3-membrane-activated PMS, which represents outstanding performance compared to catalysts reported in recent years. The influence of the PMS dosage, initial BPA concentration, and initial pH on BPA degradation were studied. The mechanism of BPA degradation in this system was investigated in depth. The BPA degradation process was not inhibited by the addition of radical scavengers to the system, and no free radicals were detected using ESR analysis. Additionally, the PMS molecules were only decomposed in the presence of both the catalysts and BPA. The results also demonstrated that only electron-rich aromatics could be degraded by this system. Furthermore, in contrast to typical degradation pathways involving the generation of free radicals, no intermediates formed via OH addition reaction were detected. Based on these findings, a nonradical pathway was suggested rather than a conventional OH/SO4− radical-based advanced oxidation process (AOP) mechanism. BPA is efficiently degraded by the transfer of electrons from its π-π aromatic ring to PMS via the noble metal Pt as transfer medium in this reaction. The findings in this work broaden the understanding of the use of activated PMS for the degradation of organic pollutants, and extend the application of flexible inorganic membranes in wastewater treatment.

Decomposition of perfluorooctane sulfonate (PFOS) using a hybrid process with electron beam and chemical oxidants

Among many contaminants of emerging concern, perfluorooctanesulfonate (PFOS) is widely known as one of the most difficult to decompose due to a very strong carbon-fluorine bond. Electron beam technology has been used to treat refractory compounds as a radical-based advanced oxidation process. Therefore, the aims of this study were to investigate the degradation efficiencies of PFOS with various initial concentrations and radiolytic products generated by PFOS decomposition using an electron beam, to improve the decomposition efficiency of PFOS using combined persulfate (S2O82−) and hydrogen peroxide (H2O2) with an electron beam, and to propose the radiolysis decomposition pathway of PFOS in aqueous solution. The results showed that the degradation efficiency of PFOS by the electron beam depended on the initial PFOS concentration. The addition of S2O82− with the electron beam increased the decomposition efficiency with increasing absorbed doses. The 0.1 mg/L PFOS (0.19 µM) in the presence of 5 mM S2O82− with the electron beam at a 100 kGy absorbed dose had a similar decomposition efficiency with that using only the electron beam at a 900 kGy absorbed dose. However, the addition of H2O2 was ineffective on the PFOS degradation in aqueous solution. Seven radiolytic products such as perfluorocarboxylic acids (PFCAs) were quantitatively confirmed using authentic standards among eight predicted radiolytic products. The radiolysis decomposition of PFOS can occur spontaneously in aqueous solution with the synergistic effects of eaq- and OH produced by electron beam irradiation. Therefore, the electron beam can be used as a viable process considering the decomposition efficiency and its degradation products.

Performance of the Persulfate/UV-C Process for the Treatment of Dimethyl Phthalate from Aquatic Environments

Phthalate esters (PAEs) are used as plasticizers to impart flexibility and resilience to plastic products. In recent years, PAEs are a controversial issue because many phthalates are suspected to be mutagens, hepatotoxic agents and endocrine disruptors, and can lead to adverse effects on organisms even in a low concentration. Recently, sulfate (SO4·-) radical based advanced oxidation processes have attracted great scientific interest due to their high efficiency in the degradation and mineralization of recalcitrant and/or toxic organic pollutants. In the present study aqueous dimethyl phthalate (DMP; 100 mg L-1), being selected as a model PAE, was treated by the persulfate (PS)/UV-C process at pH 3 and varying PS concentrations (0-60 mM). DMP and TOC abatements increased with increasing PS concentrations from 5 to 40 mM. Further increase in the initial PS concentration, however, reduced both the rate and extent of DMP and TOC removals. The highest pseudo-first-order abatement rate coefficient and electrical energy per order (EE/O) values obtained for DMP treatment with PS/UV-C oxidation were found as 0.4493 min-1 and 1.79 kWh m-3 order-1, respectively, for PS = 30 mM, pH = 3, DMP = 100 mg L-1. The second-order reaction rate coefficient for DMP with SO4·- was determined as 1.47×109 M-1s-1 by the application of competition kinetics using phenol as the probe compound. Within the scope of the present study, aqueous DMP was also subjected to peroxymonosulfate (PMS)/UV-C and hydrogen peroxide (HP)/UV-C treatments. The performance of PS/UV-C treatment was found to be higher than that of PMS/UV-C and HP/UV-C treatments both in terms of DMP and TOC abatement rates at an initial oxidant concentration of 5 mM.

Intensification of UV-C tertiary treatment: Disinfection and removal of micropollutants by sulfate radical based Advanced Oxidation Processes.

This study explores the enhancement of UV-C tertiary treatment by sulfate radical based Advanced Oxidation Processes (SR-AOPs), including photolytic activation of peroxymonosulfate (PMS) and persulfate (PS) and their photocatalytic activation using Fe(II). Their efficiency was assessed both for the inactivation of microorganisms and the removal or micropollutants (MPs) in real wastewater treatment plant effluents. Under the studied experimental range (UV-C dose 5.7-57 J/L; UV-C contact time 3 to 28 s), the photolysis of PMS and PS (0.01 mM) increased up to 25% the bacterial removal regarding to UV-C system. The photolytic activation of PMS led to the total inactivation of bacteria (≈ 5.70 log) with the highest UV-C dose (57 J/L). However, these conditions were insufficient to remove the MPs, being required oxidant's dosages of 5 mM to remove above 90% of carbamazepine, diclofenac, atenolol and triclosan. The best efficiencies were achieved by the combination of PMS or PS with Fe(II), leading to the total removal of the MPs using a low UV-C dosage (19 J/L), UV-C contact time (9 s) and reagent's dosages (0.5 mM). Finally, high mineralization was reached (>50%) with photocatalytic activation of PMS and PS even with low reagent's dosages.

Micropollutants removal by full-scale UV-C/sulfate radical based Advanced Oxidation Processes

The high chemical stability and the low biodegradability of a vast number of micropollutants (MPs) impede their correct treatment in urban wastewater treatment plants. In most cases, the chemical oxidation is the only way to abate them. Advanced Oxidation Processes (AOPs) have been experimentally proved as efficient in the removal of different micropollutants at lab-scale. However, there is not enough information about their application at full-scale. This manuscript reports the application of three different AOPs based on the addition of homogeneous oxidants [hydrogen peroxide, peroxymonosulfate (PMS) and persulfate anions (PS)], in the UV-C tertiary treatment of Estiviel wastewater treatment plant (Toledo, Spain) previously designed and installed in the facility for disinfection.AOPs based on the photolytic decomposition of oxidants have been demonstrated as more efficient than UV-C radiation alone on the removal of 25 different MPs using low dosages (0.05–0.5 mM) and very low UV-C contact time (4–18 s). Photolysis of PMS and H2O2 reached similar average MPs removal in all the range of oxidant dosages, obtaining the highest efficiency with 0.5 mM and 18 s of contact time (48 and 55% respectively). Nevertheless, PMS/UV-C reached slightly higher removal than H2O2/UV-C at low dosages. So, these treatments are selective to degrade the target compounds, obtaining different removal efficiencies for each compound regarding the oxidizing agent, dosages and UV-C contact time.In all the cases, H2O2/UV-C is more efficient than PMS/UV-C, comparing the ratio cost:efficiency (€/m3·order). Even H2O2/UV-C treatments are more efficient than UV-C alone. Thus, the addition of 0.5 mM of H2O2 compensates the increased of UV-C contact time and therefore the increase of electrical consumption, that it should be need to increase the removal of MPs by UV-C treatments alone.

UV-driven hydroxyl radical oxidation of tris(2-chloroethyl) phosphate: Intermediate products and residual toxicity

Organophosphorus esters (OPEs) are emerging contaminants widely applied as annexing agents in a variety of industrial products, and they are robust against conventional wastewater treatments. Ultraviolet-driven (UV) radical-based advanced oxidation processes have a potential to become cost-effective treatment technologies for the removal of OPEs in water matrix, but residual and newly generated toxicities of degradation products are a concern. This study is a comprehensive attempt to evaluate UV/H2O2 for the degradation of a water dissolved OPE, tris(2-chloroethyl) phosphate (TCEP). In ultrapure water, a pseudo-first order reaction was observed, and the degradation rate constant reached 0.155 min−1 for 3.5 μM TCEP using 7.0 mW cm−2 UV irradiation with 44.0 μM H2O2. Hydroxyl radicals were involved in the oxidative degradation of TCEP, as demonstrated by the quenching of the degradation reaction in the presences of tertiary butanol or ethanol. High resolution mass spectroscopy data showed a partial transformation of TCEP to a series of hydroxylated and dechlorinated products e.g., C4H9Cl2O4P, C6H13Cl2O5P and C2H6ClO4P. Based on proteomics data at molecular and metabolic network levels, the toxicity of TCEP products was reduced obviously as the reaction proceeded, which was confirmed by the up-regulated tricarboxylic acid cycle, fatty acid metabolism and amino acid metabolism in Escherichia coli cells exposed to degradation products mixture. In conclusion, incomplete hydroxylation and dechlorination of TCEP likewise are effective for its detoxification, indicating that UV/H2O2 can be a promising treatment method for OPEs removal.

Photocatalytic degradation and chlorination of azo dye in saline wastewater: Kinetics and AOX formation

Chloride ion as one of the major salt components in dyestuff wastewaters can greatly affect the homogenous OH radical-based advanced oxidation processes (AOPs), but little information is available now for heterogeneous AOPs like UV/TiO2 under similar conditions. Here the effects of chloride on kinetics and reaction intermediates of dye degradation in heterogeneous photocatalysis were examined. The dye degradation efficiency in UV/TiO2 process was investigated as a function of a wide range of salinity and pH. The chloride ion was found to have a dual effect on both the dye decoloration and mineralization in UV/TiO2 system due to different mechanisms involved. Higher Cl− concentration would inhibit dye degradation, especially in acidic medium. AOX (halogenated organic compounds adsorbable on activated carbon) generated increased with the increasing content of chloride ion present in the solution. Several toxic chlorinated byproducts were firstly identified during the dye degradation in UV/TiO2 process using GC–MS method and possible decomposition pathways were proposed. These findings may have significant technical implications for optimizing the photochemical technologies in salt-rich wastewater treatment.